1. Field of the Invention
The present invention relates to a polishing composition for selectively polishing a metal film on a semiconductor wafer and a polishing method of using the composition.
2. Description of the Related Art
A typical semiconductor wafer comprises a substrate such as a silicon or gallium arsenide wafer, and a plurality of integrated circuits are formed on the substrate. The integrated circuits are chemically or physically integrated on the substrate by patterning the regions in the substrate and layers on the substrate. The layers are generally formed of a material having any one of a conductivity, an insulating property and a semiconducting property. In order to obtain semiconductor devices at a high yield, it is very important to start from a flat semiconductor wafer. To this purpose, the surface or a certain part of the semiconductor wafer must be often polished.
According to the surface formation by conventional techniques, the upper surface of the wafer is disadvantageously left to have a topography still in a very irregular state such that upheavals, uneven height regions, troughs, trenches and other similar surface irregularities are present. Therefore, the surface must be planalized overall so as to ensure a sufficiently large depth of focus in photolithography and to remove the irregularity and surface defects during various steps in the production process. For attaining flatness on the wafer surface, several polishing methods are known and a chemical/mechanical planalization or polishing (CMP) is used over a wide range in the treatment for rendering the wafer surface flat during various steps in the production of a device, so that the yield, performance and reliability can be improved.
The slurry used for this chemical/mechanical polishing is formed from an acidic or basic solution containing abrasive grains such as silica, alumina, zirconia, ceria or titania.
The CMP technique is essential in the production process of a semiconductor device, particularly in the planalization of an interlayer insulating film for the formation of a multi-layer interconnection, in the formation of a conductive plug which is a metal, or in the formation of flush wiring.
With respect to the slurry for CMP used in the formation of a conductive plug which is a metal film comprising tungsten, aluminum, copper, titanium or an alloy thereof, JP-A-8-83780 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent application (Kokai)xe2x80x9d) discloses an abrasive composition containing a polishing accelerator such as an aminoacetic acid and/or amidosulfuric acid or an oxidizing agent, and a chemical reagent such as benzotriazole, and JP-A-8-197414 discloses a slurry containing a metal oxide particle such as fumed silica or fumed alumina having specific properties.
Conventional slurries for polishing a metal film of a semiconductor device are disadvantageous in that both the polishing rate for a metal film and the ratio (selection ratio) of the polishing rate for a metal film to the polishing rate for an oxide film (insulting layer) such as silica cannot be increased or, even if the polishing rate is high, a large number of scratches are generated on the polished surface.
The object of the present invention is to provide a slurry composition for polishing a metal film on a semiconductor substrate, which ensures a high polishing rate for a metal film, a high selection ratio, and reduction in the generation of scratches on the polished surface, as well as a method for polishing using the above composition.
As a result of extensive studies to achieve the above-described object, the present inventor has made the present invention.
More specifically, the present invention provides a slurry composition for polishing a metal film on a semiconductor substrate, comprising alumina-type fine particles containing or not containing alumina hydrate, a polishing accelerator and water, the alumina-type fine particles having an a conversion ratio of from 65 to 90% and a specific surface area of from 30 to 80 m2/g.
By the present invention, the polishing work can be favored with a high polishing rate for a metal film, particularly a high selection ratio, and reduction in scratches on the polished surface.
The alumina-type fine particles containing or not containing an alumina hydrate for use in the present invention are described below.
The alumina-type fine particles can be produced by preparing an alumina sol, forming it into a gel while heating, subjecting the gel to calcination and then firing, and crushing the fired powder in a medium stirring-type grinding machine.
To speak more specifically, a sol-form aqueous solution of a commercially available pseudo boehmite is prepared in a concentration of from 2 to 60 wt % using an inorganic acid such as nitric acid, or an organic acid, or the like. The amount of peptizer added is preferably on the order of from 1xc3x9710xe2x88x922 to 10xe2x88x925 mol/g, and the pH of the solid is preferably on the order of from 3 to 4.
If the amount of the peptizer added is less than 1xc3x9710xe2x88x925 mol/g, the peptization does not proceed satisfactorily and the fine particles after firing are deteriorated in their properties, whereas if the added amount is excessively large, the sol is abruptly gelled and the fired fine particles are also disadvantageously deteriorated in their properties.
The thus-prepared sol is dried to reduce the water content and is thereby formed into a dry gel. The gel obtained is calcined at from 500 to 800xc2x0 C. for about 1 hour and then preferably, crashed to have a gain size of about 1 mm or less.
After the calcination, the gel is fired at from 1,000 to 1,600xc2x0 C., preferably at from 1,100 to 1,300xc2x0 C., for from 1 to 3 hours. If the firing temperature is less than 1,000xc2x0 C., the xcex1-alumina conversion ratio is low and for elevating the ratio, a long period of time is necessary, whereas if it exceeds 1,600xc2x0 C., the fired fine particles may be disadvantageously sintered.
After the firing, the fine particles are preferably wet-treated by crushing or cracking in an ordinary manner, such as pot milling, to have a maximum particle size of 100 xcexcm or less.
Thereafter, the fine particles are further pulverized by a medium stirring-type grinding machine such as an attritor, a bead mill, a Dynomill, an apex mill or a co-ball mill.
The conditions for the treatment in a medium stirring-type grinding machine are as follows.
The fine particles obtained by the above-described production process are a mixture of alumina and alumina hydroxide, more specifically, a mixture of xcex1-alumina and an intermediate alumina showing a peak at 2xcex8=67.3xc2x0 at the measurement by X-ray diffraction (Cu and Kxcex1 rays), or alumina-type fine particles containing bayerite in addition to those aluminas.
The alumina-type fine particles have the following properties.
When a peak height (a) at 2xcex8=68.2xc2x0 corresponding to a-alumina and a peak height (b) at 2xcex8=67.3xc2x0 corresponding to an intermediate alumina are determined by X-ray diffraction measurement with Cu and Ka rays, the value according to the following expression, namely, the a conversion ratio is from 65 to 90%.       a    ⁢          xe2x80x83        ⁢    conversion    ⁢          xe2x80x83        ⁢    ratio    ⁢          xe2x80x83        ⁢          (      %      )        =            a              a        +        b              xc3x97    100  
The alumina-type fine particles have a BET specific surface area of from 30 to 80 m2/g.
The alumina-type fine particles preferably have a maximum grain size of 1.0 xcexcm or less, more preferably 0.5 xcexcm. If the maximum grain size exceeds 1.0 xcexcm, scratches increase on the metal film or insulating film and furthermore, the above-described selection ratio disadvantageously decreases.
In order to have a maximum grain size of a predetermined value or less, coarse particles are preferably removed by decantation or the like.
The alumina-type fine particles preferably have an average size of from 0.05 to 0.5 xcexcm, more preferably from 0.10 to 0.30 xcexcm, the most preferably 0.15 to 0.30 xcexcm.
If the average size is less than 0.05 xcexcm, the mechanical polishing force decreases and the metal film can be difficult to shave, whereas if it exceeds 0.5 xcexcm, the polishing force increases but generation of scratches on the metal film or insulating film increases.
The concentration of the alumina-type fine particles in the polishing composition of the present invention is preferably from 0.5 to 20% by weight, more preferably from 1 to 10% by weight. If the concentration is less than 0.5% by weight, the polishing rate decreases. If the concentration exceeds 20% by weight, the polishing rate does not increase and the selection ratio decreases. Particularly, a range of from 1 to 10% by weight is optimum from the viewpoints of the polishing rate and the selection ratio.
The xcex1 conversion ratio of the alumina-type fine particles of the present invention is optimum in a range of from 65 to 90%. If the a conversion ratio exceeds 90%, the scratches formed on the polished surface increase. If the xcex1 conversion ratio is less than 65%, the polishing rate decreases. Outside of the range is not preferred.
The polishing accelerator for use in the present invention is described below. The polishing accelerator which can be used in the present invention is not particularly limited, however, Fe(NO3)3 type or H2O2 type oxidizing agents, inorganic polishing accelerators such as ammonium cerium(IV) nitrate, ammonium cerium(IV), sulfate, cerium(IV) sulfate and cerium(IV) chloride, organic polishing accelerators such as organic acids having a molecular weight of 100 to 300 of carboxylic acid, or the like are preferred.
When the inorganic polishing accelerator is used, the amount of the accelerator added is preferably from 2 to 7% by weight based on the polishing composition. If the added amount is less than 2 wt % high-speed polishing becomes difficult, whereas if it exceeds 7 wt %, the effect due to increase in the added amount is not increased and this is not preferred in view of profitability. In the case of the organic polishing accelerator, the amount of the polishing accelerator added is preferably from 0.1 to 5% by weight based on the polishing composition.
The slurry of the present invention may contain various known additives commonly added to the polishing slurry composition, such as a dispersant, a buffer, a surface active agent and a viscosity controlling agent, within the range of not impairing the preferred properties of the slurry.
The polishing composition of the present invention may be preferably used for polishing a metal film on a semiconductor substrate. The metal film includes tungsten, aluminum, copper, titanium, tantalum, etc., and alloys of these metals. The polishing composition of the present invention provides a high polishing rate of a metal film. The polishing composition of the present invention is particularly useful in selectively polishing a metal film of a semiconductor substrate having a metal film and an insulating film thereon. The insulating film may be an oxidized silicon or nitrized silicon film. The polishing composition of the present invention provides a high selection ratio of polishing a metal film to polishing an insulating layer, for example, above 200, even above 400 for W/SiO2, above 600 for Cu/SiO2, and above 800 for Co/SiO2.
For example, when a metal film is formed on an insulating layer as above, which in turn is formed on a semiconductor substrate, since the polishing selection ratio between a metal film and an insulating layer is high in the polishing composition of the present invention, the finish point of polishing a metal film can be easily detected and controlled without excessive polishing of the underlying insulating layer.